1. Field of the Invention
The present invention relates to a sheet conveying apparatus used with an image forming apparatus or an image reading apparatus such as a copying machine, a scanner, a printer and the like. More particularly, it relates to a sheet conveying apparatus having a skew correction means for correcting skew-feed of a sheet conveyed to an image forming portion or an image reading portion.
2. Related Background Art
In some conventional image forming apparatuses and image reading apparatuses such as copying machines, a printers or scanners, a regist means acting as a skew correction means for correcting skew-feed of a sheet is disposed in front of an image forming portion or an image reading portion in order to correct posture and position of a sheet.
Among such regist means, there is a loop regist means in which a tip end of a sheet abuts against a nip between a pair of regist rollers which are now stopped to form a loop in the sheet, so that skew-feed of the sheet is corrected by aligning the tip end of the sheet with the nip by elasticity of the sheet. As another regist means, there is a shutter regist means in which a shutter member for stopping the tip end of the sheet is retractably disposed in a sheet convey path and the skew-feed of the sheet is corrected by retarding the shutter member from the sheet convey path after the tip end of the sheet is aligned with the shutter member.
Recently, as the image forming apparatus and the image reading apparatus have been digitalized. For example in the image forming apparatus, a substantial image forming speed has been increased by treating many sheets for a short time without increasing a process speed of image formation by decreasing a distance between the sheets (sheet interval). On the other hand, in conventional analogue apparatuses (for example, copying machines), even when a copying operation is continued after a single sheet (original) is read, an optical device for exposing the original must be reciprocated by times corresponding to the number of copies, so that the distance between the sheets (sheet interval) is determined accordingly.
However, since the image reading and the image formation are digitalized, after the original is read once, image information of the original is electrically can be coded to be stored in a memory. And, in the image formation, the information in the memory is read out, and an image corresponding to the image information of the original is formed on a photosensitive member by an exposure device such as laser light or an LED array. To this end, even when a plurality of copies are formed, a mechanical movement of the optical device is not required.
As a method for reducing a time for the abovementioned registration which is one of factors for determining the distance between the sheets (sheet interval), there has been proposed an active regist method for correcting the skew-feed of the sheet while conveying the sheet without stopping the sheet temporarily.
In this method, two sensors are disposed in the sheet convey path with a predetermined distance therebetween along a direction substantially perpendicular to the sheet conveying direction so that inclination of the sheet can be detected on the basis of signals representing the fact that the tip end of the sheet is detected by the respective sensors, and, by controlling sheet conveying speeds of a pair of regist rollers which are disposed coaxially in a direction substantially perpendicular to the sheet conveying direction and spaced apart from each other with a predetermined distance therebetween and which are driven independently, the skew-feed of the sheet is corrected. By effecting the skew correction without stopping the sheet temporarily in this way, the distance between the sheets (sheet interval) can be reduced more than the other methods.
However, in the above-mentioned conventional sheet conveying apparatus, and the image reading apparatus and the image forming apparatus having such a sheet conveying apparatus, when sizes of sheets to be conveyed are not constant or identical (particularly, when a sheet having a long size is conveyed), the skew correction should be effected by the pair of regist rollers while a trail end of the sheet is being pinched between a pair of upstream convey rollers.
Further, in the active regist method, the skew correction is effected by advancing delayed side of the sheet with respect to one of the pair of regist rollers for skew correction or by delaying advanced side of the sheet with respect to the other of the pair of regist rollers. However, in both cases, rotational movement of the entire sheet is required. Thus, in the condition that the trail end of the sheet is pinched between the pair convey rollers, it is difficult to rotate the sheet by a required amount, which makes the accurate skew correction difficult. Further, depending upon the size of the sheet, sliding resistance of a sheet convey guide is increased to worsen accuracy of the skew correction.